The present invention relates to electric storage batteries. More particularly, the invention relates to a novel electric storage battery with an iron salt as cathode.
There is an ongoing need for providing novel improved electrical storage batteries, which are low-cost, have a high-energy density and are environmentally acceptable. Among the main types of storage batteries are those in which the cathodes (the positive electrodes) are based on any of PbO2, HgO, MnO2 and NiOOH which are known to possess a theoretical capacity in the range of between 224 to 308 Ah/g. However, these cathode materials are considered as hazardous or environmentally unfriendly.
In a very recent U.S. Pat. No. 5,429,894, iron-silver (iron in its zero valence state) was suggested as a battery anode (negative). Iron salts in the +2 and +3 valence state, were also suggested as a battery cathode in the past as described, for example, in U.S. Pat. No. 4,675,256 and U.S. Pat. No. 4,795,685.
Prima facie, salts containing iron in the +6 valence state, hereafter called Fe(VI), which are capable of multiple electron reduction, would be capable to provide a higher cathode storage capacity. However, decomposition with reduction of the iron to a less oxidized form (i.e. to a lower valence state) occurs very rapidly, the stability of Fe(VI) salt solutions being only the order of a few hours at room temperature (Anal. Chem. 23, 1312-4, 1951). The Fe(VI) salts may be made by chemical oxidation, such as reported by G. Thompson (J. Amer. Chem. Soc. 73, 1379, 1951), or by precipitation from another Fe(VI) salt, such as reported by J. Gump et al. (Anal. Chem. 26, 1957, 1954). However, as mentioned in a later report by H. Goffet al (J. Amer. Chem. Soc. 93, 6058-6065, 1971), only little is known on the chemistry of Fe(VI) salts. The decomposition of an Fe(VI) salt to a salt in which the iron has a lower valence, results in a spontaneous loss of the electrochemical storage capacity. For example, the anion FeO4xe2x88x922 such as in K2FeO4, is unstable in neutral aqueous solutions and decomposes at a rate kf according to the following equation:
2FeO42xe2x88x92+3H2Oxe2x86x922FeOOH+3/2O2+4OHxe2x88x92
The resultant product in this decomposition, Fe(III)OOH, is environmentally more friendly than any of PbO2, HgO, MnO2 and NiOOH, but has a lower electrochemical storage capacity.
It is an object of the present invention to provide a novel type of battery which is inexpensive, highly stable, possesses a high storage capacity, a high voltage and is environmentally friendly, a battery using additives which can further improve the electrochemical characteristics of the battery.
The invention relates to an electrical storage cell, so-called battery, comprising two half-cells which are in electrochemical contact with one another through an electrically neutral ionic conductor, wherein one of said half-cells comprises an anode and the other half-cell comprises a cathode in form of a solid-phase Fe(VI) salt in an amount of at least 1% of the half-cell weight, whereby electrical storage is accomplished via electrochemical reduction to a valence of iron salt less than Fe(VI). The high +6 valence state of the iron in said salt provides the advantage of a high storage capacity and high voltage, and iron salts provide an environmental advantage over more toxic materials used for electrochemical electric storage. Small material additions to the Fe(VI) salt containing half cell, and to the electrically neutral ionic conductor can further improve the electrochemical characteristics of the battery.